1. Field of the Invention
The present invention pertains to an optical-fibre bidirectional telecommunication system and to a bidirectional optical amplifier.
In recent times the use of optical fibres to send optical signals carrying the information to be remotely communicated has become well known in the telecommunication field.
Also known is the fact that optical signals sent over an optical fibre are submitted to an attenuation during their travel, which will make it necessary to amplify the signal so that it may cover the whole intended distance and reach the receiving station with a sufficient power level for a correct reception of the forwarded messages.
This amplification can be carried out by appropriate amplifiers located at predetermined intervals along the line, which amplifiers periodically raise the power of the transmitted optical signal.
For the purpose, optical amplifiers are conveniently used by which the signal is amplified while remaining in an optical form, that is in the absence of an optoelectronic detection and an electro-optical regeneration of same.
Such optical amplifiers are based on the properties of a fluorescent dopant (erbium for example) that, if suitably excited by administration of luminous energy, gives a high emission in the wavelength band corresponding to the minimum light attenuation in the silica-based optical fibres.
Such amplifiers are apparatus of the unidirectional type, that is in which the optical signal has a predetermined travel direction. This is due, as disclosed for example in U.S. Pat. No. 5,204,923 and U.S. Pat. No. 5,210,808 of the assignee of this applicant to the fact that optical amplifiers, in particular where high gain values are required, incorporate components of the unidirectional type, adapted to prevent signals reflected externally of the amplifiers from coming back to the amplifier itself, due for example to the Rayleigh scattering along the optical line fibres connected to the amplifiers.
As a consequence, a bidirectional transmission requires the use of two separate communication lines provided with respective amplifiers, each of which is used for communication in a single direction, which will result in a high cost for connection.
2. Related Art
Attempts have been made to achieve a bidirectional amplification with the use of a single unidirectional amplifier by exploiting the possibility offered by fluorescent-dopant amplifiers to amplify signals at different wavelengths in an independent manner. A bidirectional amplifier based on this principle is described in an article by S. Seikai et al.: "Novel Optical Circuit Suitable for Wavelength Division Bidirectional Optical Amplification" published in Electronics Letters, Vol. 29, No. 14, Jul. 8, 1993, pages 1268-1270. It is an apparatus which is located along an optical-fibre transmission line in which two signals of different wavelengths propagate in opposite directions and which consists of wavelength-selective couplers and a unidirectional doped-fibre amplifying unit of known type, connected to each other by portions of passive optical fibre. Both of the signal wavelengths are internal of the doped-fibre amplifying band. Through the selective couplers the two signals of different wavelength are admitted to different optical paths of travel. The two optical paths of travel only coincide at the portion corresponding to the amplifying fibre which is travelled over by the two signals in the same direction. The apparatus that will be described in more detail in the following, has a problem in terms of instability caused by internal reflections at an intermediate one of the wavelengths of the propagating signals, which problem can be only solved by the addition of filters, some of them being adjustable. As a result a very complicated structure is required and it is necessary to use devices for a continous and precise adjustment of said filters.